Transition from plow to plane

Observing RPM, MPH, GPH, and MPG data on planing power boats there is often a significant jump in speed when transitioning from plowing to planing. Throughout the plowing spectrum speed and RPM's are generally linear, and likewise when planing. For instance, for every 500 RPM increase, speed increases by around 2-5 MPH. But there is a point where the same incremental increase in RPM's, e.g. 500, takes the boat from plow to plane and the speed increases exponentially, say 15 MPH. I understand why this is the case, but am wondering what the practical implications are when sea trialing a boat for different purposes. For example, in some situations, e.g. heavy seas, the optimal speed might be 15 MPH, but a particular boat may go from 10 MPH at 3,000 RPM's plowing, to 25 MPH at 3,500 RPM, up on plane, making it impossible to hold the speed and have control of the vessel at 15 MPH.

Has this been discussed anywhere in this forum? Has anyone found any particular types or combinations of hulls, deadrise, power-to-weight ratios, or power-to-length ratios, beam, etc. that minimize this effect?

Has anyone found any particular types or combinations of hulls, deadrise, power-to-weight ratios, or power-to-length ratios, beam, etc. that minimize this effect?

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This is what every seriously working designer works with all the time. It is important to optimize the various input parameters against each other so that the result is optimal, no compromises.
No guesses, just calculations.

Last edited: Feb 15, 2019

www.sassdesign.net
I'm not lost, I'm just uncertain of my position.
I'm still confused, but on a higher level

It sounds like you are interested in the "hump" in resistance as a boat starts to plane. The magnitude of the hump is strongly influenced by the weight of the boat relative to it's length. With similar hull shapes a heavier boat for it's length will usually have a larger hump. Another factor is the longitudinal position of the center of gravity of the boat. The further aft the center of gravity the larger the hump. My impression is lower deadrise angle typically results in a smaller hump if everything else is "equal". Power does not influence the magnitude of the hump but the magnitude of the hump may be a factor in deciding on how much power to install.

The difference in the boat speed to engine speed relationship has to do with prop slip... At low speeds and loads there isn't much slip since the loading is low and the prop not doing much work. As the speed and load increase and the boat approaches hump speed slip goes up because the loading is high and the speed is still low. Once you get over the hump and speed jumps, the amount of water that is passing through the prop disk area (think of the prop swept area as a disk), is higher so the prop loading (surface pressures on the blades) drops back down and slip is decreased. So what you are seeing is really just slip. In a perfect world with no slip, the engine speed would correlate with boat speed from idle to top speed. In the hump range, prop slip is at its highest value because of the high load and the lower boat speed. Now if you change props, or move the CG forward, or trim the motor differently, or add or change trim tab position, you can change the power demand vs speed relationship, and change the amount of engine speed vs forward speed you observe. But what you are seeing is really just slip and the amount can be calculated based on pitch and engine rpm at a given speed and compared that with the theoretical speed for the same pitch and rpm. If you take that data you'll see slip going to a maximum near the hump speed and then dropping off as you go faster than that.

I think he probably realises it is a high resistance stage for a planing boat, but wants to smooth out the curve a bit. It is not a bad idea to run a slightly lower pitch, and maybe more prop blades, if routinely running at backed-off speeds. Failing that, get a good power cat, and backing off is rarely necessary !

Thanks all. Great input and you all have provided the context I was looking for to do further research and decide on my next boat. I am starting to compile a lot of performance data across different manufacturers, like Grady White and Robalo, or whatever I can get the data on (GW posts all of their performance data on line for each model and various engine set ups), and a few different styles of boats, primarily Center Consoles and Express Cabins, because I think I would prefer a CC but the wife wants the option to get out of the weather. We intend to do mostly coastal cruising in the FL Gulf Coast area, less than 20 miles off shore, unless I decide to head to Key West or Bahamas, or do the Great Bend some day, in which case I may just want a different boat... All that said, so far I like to profile of the GW Canyon 376 with triple Yamaha 300's as follows: 2500 RPM: 11.8 MPH; 3000 RPM: 20.4 MPH; 3500 RPM: 29.1 MPH. So far most boats I have looked at are pretty consistent before and after the hump/plane with one big jump in speed of around +15 MPH at the 3000-4000 RPM area. You're either doing 10 MPR or 25 MPR but you can't go 15 MPR. But the Canyon 376 has two small bumps in speed, so like what Mr. Efficiency said, it smooths out the curve a bit. This is all very preliminary, but I wanted to give you all more insight into what I'm trying to accomplish and what I'm looking for in a boat. And while the Canyon 376 looks like an awesome boat, it's really out of my price range. I'm trying to stay around the $150,000 budget. When did boats start costing as much as airplanes? Cheers!

The graphs you see of speed vs rpm, are going to be different, depending on the trim angles of the engines, motors trimmed well in, will go a long way toward smoothing out the "hump", there is also the variability imposed by what direction to the sea you are running, to consider, and maximum in-trim would be undesirable, if running "downhill", but you should not need to be backed-off so much under those conditions, anyway, with a good hull design. If you are unable to use maximum in-trim with both twin engines, because you need differential trim to level the boat level, then tabs should also be used. It is well known that most planing boats have a speed band where they are not at their best, and it may at times, be the maximum speed that sea conditions allow, but generally speaking , recreational users will not be at sea when those conditions prevail.

I think he is interested in boats with more speed than those warped plane boats, when you combine deep vees with substantial weight, the "hump" is going to be there. Planes don't take off and land, climb and cruise etc, without changes in wing geometry, in a planing boat you can change geometry with tabs and interceptors, and the luxury of being able to trim engines to change the line of thrust, and especially with engines perched out on pods, that needs to be taken full advantage of, but if we are talking 15 knot kinds of speeds, your typical heavy deep vee will still not be ideal. Boats are typically designed for the "average" range of conditions, they are likely to be used in, you can't optimize for all speeds, in the one boat.

Quick update, I have all but decided on a Robalo R305, twin Yamaha 300's, walk around. Seems to be a good balance of offshore ride and topside/cabin comfort. No such thing as a perfect boat...

Having said that, I did come across the Cutwater boats which are pretty interesting. Something between a trawler and a cruiser. They apparently plane at 12-13 knots, but top speeds are around 28 knots. So it solves my problem above with the "hump" issue, capable of slow cruising in rough seas, but I find that to be the case with displacement hulls in general.

There I have to make the decision - go with a 40 foot (bigger is better), diesel powered (fuel efficiency), displacement hull (slow and steady wins the race), where one can cruise just anywhere (except the shallows where you need a bay boat), under most weather conditions; or a sedan type boat (floating condo) that gulps gas, cruises at 30, will handle worse seas far better than the crew will be able to, and may be a rough ride in that fowl weather, but much more amenities for nice weather - which should be 90% of my boating.

Another tidbit, interesting to see that the planing hulls get about the same fuel efficiency at displacement speeds as the trawlers do - did not expect to see that. However, they obviously aren't made to cruise at that speed and therefore are not optimally designed for handling at that speed.

Being new to the gulf, west coat Florida boating, I am at a knowledge/experience deficit so I'm not sure what boat I will ultimately end up with. We don't intend to shotgun beers on the sand bars and we don't intend to do hardcore offshore fishing 100 miles out. Something in between - cruising the barrier islands between Tampa and Ft. Lauderdale, maybe an occasional overnight, but like to idea of having A/C when it gets unbearably hot.

Any additional input from you guys (and gals) would be greatly appreciated.

Another tidbit, interesting to see that the planing hulls get about the same fuel efficiency at displacement speeds as the trawlers do - did not expect to see that. However, they obviously aren't made to cruise at that speed and therefore are not optimally designed for handling at that speed.

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Are you comparing boats of the same weight? Also which "trawler" design? The hulls of some "trawlers" appear to be optimized for interior volume and accommodations rather than low drag.

Sorry, not really a serious comparison. Both are designed for different performance criteria. Just mildly amused that, like a trawler, a planing hull can get 2 MPG at 8 knots, but a trawler can’t do 40 knots. I suppose if this were a more serious comparison I would research planing hulls that were known for good handling at displacement speeds. Do these exist?

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